window-panes. In other cases they unite in radial groups about certain centers, and thus build up globular masses to which the name of "spherulites" has been given.
In Figure 1 No. 1 represents a glass through which microliths or crystallites of different dimensions and character are diffused. In
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/38/PSM_V20_D363_Sections_of_igneous_rocks.jpg/400px-PSM_V20_D363_Sections_of_igneous_rocks.jpg)
Fig. 1.—Sections of Igneous Rocks, illustrating the Passage from the Glassy to the Crystalline Structure. 1. Vitreous rock. 2. Semi-vitreous rock. S. Vitreous rock with spherulites. 4. Rock with cryptocrystalline base. 5. Rock with microcrystalline base. 6. Rock of granitic structure built up entirely of crystals.
Nos. 2 and 3 the crystallites have united to form regular groups. No. 4 represents a glassy ground-mass containing microliths (a crypto-crystalline base), through which distinct crystals are distributed, and is typical of the features presented by most lavas. Nos. 5 and G illus-